The goals of this project are to resolve functionally specialized subpopulations of B lymphocytes, determine the developmental relationships between them, and probe the inductive interactions necessary to their formation from multipotential stem cells. Mitogen-dependent functional assays, physical separation techniques, and cell surface markers are being used to assess B-cell heterogeneity, establish precursor-product relationships, and detect emerging B cells in cultures of immature tissues. It is now possible to obtain cell suspensions which contain pluripotent hematopoietic stem cells but lack cells with detectable B or pre-B characteristics. Also we have developed a functionally and cytogenetically marked system for distinguishing the progeny of these stem cells from cells derived from putative inductive microenvironments. With these techniques we hope to identify the cellular components of such bursal equivalent structures, locate them within various tissues, and explore the role of humoral and contact mediated events in the transition of stem cells to functioning B cells.